1. Field of the Invention
The invention relates to treatment of ores, particularly phosphate ore. More particularly, it relates to modifying the process of formation of dispersed clays (slimes) therein. Still more particularly, the invention relates to the modification of dispersed clay formation in an ore matrix, particularly in a phosphate ore matrix, by dispersing the matrix in a dilute aqueous solution of ethylendiaminetetraacetic acid (EDTA) prior to ore beneficiation, whereby the sedimentation and consolidation characteristics of these by-product clays is greatly improved.
2. Discussion of Prior Practices and Publications
The preferred phosphate matrix is found in Central Florida and in other places and occurs in large deposits averaging about 20 feet in thickness. Since the matrix deposit begins about 15 feet below the surface, recovery is by open pit methods.
In addition to the phosphate rock and silica, the matrix contains a substantial amount of clay, which is predominantly less than 1 micron in size. It is this clay that presents one of the substantially unsolved problems of the ore mining industry.
After the phosphate matrix is removed from the open pit by a dragline, or by other means, it is dropped into a sluice pit, where it is slurried with water and transported through pipelines to the beneficiation plant. During hydraulic slurrying, transporting and subsequent washing, screening and classification, the clay particles are dispersed in the water, producing the so-called phosphatic clay suspensions or slimes.
In the conventional phosphate beneficiation process, tremendous quantities of clay suspensions are formed, amounting to about 20-40% of the phosphate ore mined. When it is considered that these suspensions are formed as aqueous suspensions containing about 1-5% solids, one can readily appreciate the enormous quantities of water used. It is the disposal of the vast amounts of slimes generated and secondarily the recovery of phosphate values from slimes that present perhaps the most pressing problems that face the phosphate industry, and that have faced it for a long time.
Clay suspensions present a problem because they retain substantial amounts of their original water, even after years of settling. Not only does this result in an area having no bearing strength, but it also results in the waste of increasingly rare natural resources, water and land. In this regard, even after settling for many years, the suspensions settle to only about 20-25% by weight of solids and still are jelly-like in consistency. So, considering this, it is no wonder that there are such large areas near the phosphate producing areas of Florida that are virtually useless for any purpose.
Because of this, dams are necessary to retain the suspensions, and they must be continuously inspected and repaired. Such dams have been known to fail, and as a result to pollute the land, rivers and lakes. This is especially severe in the case of polluted waters because of the potentially lethal effects on fish and other life.
The industry has attempted over the years to eliminate the wasteful loss of phosphate values, water and land. Such efforts are so numerous that no purpose would be served by enumerating all of them, but a few will be mentioned as illustrations.
Attempts have been made to coagulate the clay suspensions by mechanical methods (e.g., U.S. Pat. No. 3,008,575) and chemical methods (e.g., U.S. Pat. No. 3,314,537), but such attempts have been made after the suspensions have formed. One U.S. Pat. No. (4,194,969) discloses a method for treating the ore matrix prior to classification. The patent discloses that certain water soluble metal salts, the metal being aluminum, calcium, magnesium or zirconium, can be added to the ore matrix to modify phosphatic clay formulation. It further discloses the addition of such salts to the ore matrix prior to screening, sizing and the like. However, none of these methods teach or suggest the present method, including the efforts disclosed in the preceeding three patents.